The present invention relates to an OFDM (orthogonal Frequency Division Multiplex) receiver such as a digital terrestrial broadcasting receiver or the like, and particularly to level control on a received signal converted into an intermediate frequency by a tuner.
FIG. 2 is a block diagram showing the outline of a conventional digital terrestrial broadcasting receiver.
The digital terrestrial broadcasting receiver has an antenna 1 which receives an OFDM-modulated wireless or radio signal RF lying in an UHF (Ultra High Frequency) band, and a tuner 3 which frequency-converts the radio signal RF received by the antenna 1 in accordance with a local oscillation signal LO of a local oscillator 2 to thereby generate an intermediate frequency signal IF for a desired reception channel. The intermediate frequency signal IF is amplified in such a manner that average power becomes a constant value by a variable gain amplifier (hereinafter called “AMP”) 4 whose amplification is controlled by a gain control signal AGC, after which it is supplied to an analog-digital converter (hereinafter called “ADC”) 5. A fast Fourier transformer (hereinafter called “FFT”) 6 and a power calculator 7 are connected to the output side of the ADC 5.
The FFT 6 converts a time-domain signal converted to a digital value by the ADC 5 to a frequency-domain signal corresponding to a plurality of carriers constituting OFDM. Although not shown in the drawing, an equalizing unit which generates receive data in sync between the plurality of carriers, an error correction unit, a video/audio reproducing unit, etc. are connected to the output side of the FFT 6.
On the other hand, the power calculator 7 calculates a value corresponding to the average power of the output signal of the AMP 4, based on the time-domain signal converted to the digital value by the ADC 5. The output signal of the power calculator 7 is supplied to a digital-analog converter (hereinafter called “DAC”) 8, where it is converted into an analog signal, followed by being supplied to the AMP 4 as a gain control signal AGC.
In the digital terrestrial broadcasting receiver, a wireless or radio signal RF lying in a range of 450 MHz to 700 MHz, which has been received by the antenna 1, is converted into an intermediate frequency signal IF whose band is about 450 kHz with its center frequency as about 500 kHz by the tuner 3. For example, 108 carriers are multiplexed into the intermediate frequency signal IF. Each modulated wave has been quadrature-modulated based on data constituting broadcast contents and a control signal. The intermediate frequency signal IF outputted from the tuner 3 is amplified to a predetermined level by the AMP 4, followed by being supplied to the ADC 5, where it is converted into a digital value in accordance with a sampling clock of 2 MHz, for example. The received signal converted into digital form by the ADC 5 is supplied to the FFT 6, where it is separated into signals set every carrier, thereby generating receive data, after which they are reproduced as images and sound.
Further, the received signal converted into the digital value by the ADC 5 is supplied to the power calculator 7, where average power set for a predetermined period (e.g., 1 symbol period corresponding to modulation unit of carrier=about 1 ms) is calculated. The value of the average power calculated by the power calculator 7 is converted into an analog gain control signal ACC by the DAC 8, followed by being supplied to the AMP 4. In the AMP 4, its amplification is reduced when the gain control signal AGC becomes larger, whereas when the gain control signal AGC becomes smaller, its amplification increases. Thus, the average power of the output signal of the AMP 4 converges to a predetermined value.
Accordingly, the received signal suitably converted into the digital value is obtained from the ADC 5 by setting the average power outputted from the AMP 4 so as to reach the optimum input level of the ADC 5.
The above related art refers to a patent document 1 (Japanese Unexamined Patent Publication No. 2004-153811).
However, the following problems arise upon calculation of the average power by the power calculator 7.
That is, there is a need to calculate instantaneous power and calculate its average value upon calculation of average power. Since the instantaneous power is proportional to the square of the voltage, it is necessary to perform a square calculation of the digital value outputted from the ADC 5 in accordance with a sampling clock. Since the range of the digital value outputted from the ADC 5 is of a relatively wide-ranging value, the number of digits for the squared result becomes large. Further, there is a need to accumulatively add the values of respective instantaneous power for one symbol period (about 1 ms) for the purpose of determining the average power. Therefore, the problems arose in that there was a need to perform a high-capacity calculation at high speed, and the scale of the power calculator 7 would increase.